1. Field of the Invention
The invention generally relates to an apparatus and method for the manufacture of high quality communication cables of the type including a single or a plurality of sets of twisted wires.
2. Description of the Prior Art
Communication cables of the type which include a plurality of twisted wires are manufactured in either one stage or in two stages.
In the case where cables are manufactured in two stages, the twisted wires are firstly prepared by twisting the wires together by means of so-called twinning or pairing machines. Twisted wires are then made up into communication cables by means of, for example, stationary take-ups, rotating take-ups (also called drum twisting machines) or other type of rotating equipment.
One form of equipment conventionally used for twisting 2, 3 or 4 wires is the double twist machines. The resulting twisted elements are called pairs, triads or quads.
This equipment includes a bobbin cradle around which is arranged a rotatable frame or bow which is driven to turn around the cradle. Wires to be twisted may be supplied from bobbins on the bobbin cradle inside the twisting cage and taken up on a take-up reel outside the twinning cage. The aforementioned arrangement is referred to as an "inside-out" machine. The wires to be twisted may also be supplied from outside the twisting cage and taken up on a bobbin arranged within the bobbin cradle. The latter configuration is sometimes referred to as "an outside-in machine."
Outside-in machines are generally preferred in individual twisting machines since the wire may be supplied from storage facilities of simple construction and greater capacity. In this case, the bobbin cradle within the twisting cage is also required to hold only a single bobbin. The outside-in machine is also readily adaptable for use with a greater number of wires.
If communication cables are made in one stage, the apparatus generally employs a plurality of twisting machines, or heads of the "inside-out" type.
The twisted elements so manufactured are directed to any type of take-up (e.g. stationary or rotating take-ups, single or double twist machines, capstan or extrusion lines) for laying up twisted wires to form a communication cable. This is done in one operation.
The plurality of double twist twisting machines can be arranged horizontally or vertically, depending on the preferred plant layout.
One typical example of such an installation is disclosed in U.S. patent application Ser. No. 08/163,735 assigned to the assignee of the subject application.
It is well-known in the art that the lay obtained with double twist actions is not perfectly regular and if longer lays are used, in order to achieve higher speed of manufacture, some irregularity in the position of the cores in the twisted elements have to be accepted.
These irregularities in the lays do not cause problems in communication cables such as low frequency telephone cables used in standard telephone applications since the perfect constancy of the lays and in the relative position of the individual wires in each element (pair, triad or quad) are not that important.
With the advent of high speed data transmission, especially for computer use and other applications, the frequencies required are much higher and therefore standard pairs, triads or quads acceptable in telephone networks cannot be used in such high frequency applications.
In order to produce pairs, triads or quads that can operate satisfactorily at these frequencies, it is necessary to produce a cable in which the elements of each pair, triad or quad are maintained in the same desired position relative to each other so that the electrical characteristics of the pair, triad or quad do not vary along the length of the cable.
It is well-known, for example, that the characteristic impedance of an n-wire line is a function not only of the diameters of the individual conductors but also a function of the spacing or distances between the conductors. Matched impedances are critical at high frequencies to optimize power transfer, reduce line reflections which cause deterioration of signal integrity and optimize the useful frequency for which the cable can be used.
It has been proven that for example, the characteristic impedance of pairs can change drastically at different frequencies around its theoretical average. Cables utilizing high quality pairs have been produced for use in communication local area networks with a maximum useful frequency of 100 MHz. This, in the industry, is called a Level or Category 5 cable. The specification for these cables requires, for example, that the theoretical characteristic impedance of 100 Ohms can only vary between 85 and 115 Ohms from Zero to 100 MHz.
This can only be achieved by assuring that the relative position of each element is maintained throughout the length of the cable. The acceptable way of achieving this has been to shorten the lays of the elements in order to manufacture a mechanically more stable element. This approach has naturally reduced the productivity of the equipment used since there are physical limitations to the rotational speed of the bows used in the double twist machines.
The industry is already requiring twisted elements, especially pairs, that will maintain their electrical characteristics up to 350 MHz; this is normally called a Level or Category 6 communication cable.